1. Field of the Invention
The present invention relates to a laminography system for obtaining an image of lamina of a test object, thereby inspecting interior of the test object, which are not observed with the naked eye.
2. Description of the Prior Art
Generally, various quality inspections are carried out during the manufacturing of industrial products in order to confirm whether the products are acceptable or not. Among these, a laminography system is utilized for the inspection of certain areas hardly observed with the naked eye, such as solder connections of electronic devices mounted on a printed circuit board, or the like.
Construction and operation of a prior art laminography system is described hereinbelow.
As shown in FIG. 1, a laminography system 1 includes a table 10, an X-ray source 20, an image intensifier 30, a view selector 40, and a camera 50. X-rays irradiated from the X-ray source 20 rotate along the circumference of the X-ray source 20 with a predetermined speed. Such rotating X-rays are transmitted through a test object E which is loaded on the table 10. The X-rays transmitted through the test object E produce images on an image projecting plane 32.
At this time, due to the different positions from which the rotating X-rays are irradiated, the images are projected onto different portions of the image projecting plane 32 as shown in FIG. 2. The images on the respective portions of the image projecting plane 32 are then interposed on one another, and an image of a lamina is obtained.
Generally, due to the rotation of the X-rays along the circumference of the X-ray source 20, the image transmitted through the test object E rotates on the image projecting plane in the circumferential direction thereof. Therefore, the images are selected in order by the view selector 40, and received into the camera 50.
The above-described view selector 40 guides the images from the respective areas of the image projecting plane 32 into the camera 50 by means of a pair of mirrors. U.S. Pat. Nos. 4,926,452 and 5,594,770 disclose typical examples of such view selector.
According to the view selector of U.S. Pat. No. 4,926,452 as shown in FIG. 3, a pair of mirrors 42' and 44' facing each other are rotated together with the images rotating along the circumference of the image projecting plane 32, so that the mirrors 42' and 44' guide the images into the camera 50. Here, the mirrors 42' and 44' and images are rotated together at the same speed. Meanwhile, FIG. 4 shows the view selector of U.S. Pat. No. 5,594,770 in which a pair of mirrors 42" and 44" are disposed under the image intensifier in such a manner that the mirrors 42" and 44" can be rotated respectively about an X-axis and an Y-axis. Thus, when the images are moved on the image projecting plane 32 disposed on the image intensifier, the mirrors 42" and 44" are rotated about the X, and Y-axes so as to select the image of a certain area. Accordingly, as the images on the image projecting plane 32 are reflected on the mirrors 42" and 44" in subsequent order, the images are sequentially received into the camera 50. The reference numerals 43" and 45" in FIG. 4 denote motors for rotating the mirrors 42" and 44".
According to the view selector of U.S. Pat. No. 4,926,452, however, the view selector is operated only when the images are moving in the circumferential direction of the image projecting plane. Further, the view selector of that patent has a drawback in that the images at the center portion of the image intensifier can not be easily obtained. The view selector of U.S. Pat. No. 5,594,770 has the possibility of having image distortions due to the fact that the angle of incidence of the X-rays keeps changing in accordance with the different positions of the images.